**2.4 Analytical methods**

COD concentration was spectrophotometrically analyzed using a HACH spectrophotometer and methods as in Spectrophotometric Instrument Manual. Gas collection was done using daily water displacement. Methane content was analyzed using gas chromatography with thermal conductivity detector (GCTCD) with helium as the carrier gas. Acetic acid concentration (TVA) was determined using HPLC. Substrate concentration was measured as suspended solids according to the Standard Methods for The Examination of Water and Wastewater. 20 ml well-mixed sample was filtered through a weighed standard glass-fiber filter and the residue retained on the filter is dried to a constant weight at 103oC to 105oC. The increase in weight of the filter represents the total suspended solids (APHA 1989).

#### **3. Results and discussion**

Chen et al. (1978) developed a kinetic model on substrate utilization based on the Contois model as follows:

$$\frac{\mu\_{\text{max}}}{\mu} = K \frac{\text{S}\_{\text{o}} - \text{S}}{\text{S}} + \mathbf{1} \tag{18}$$

The kinetic model has been aptly used in many studies, notably in investigations on anaerobic digestion of high strength wastes (Mata-Alvarez et al. 1992; Sales et al. 2000).

Equation (18) can be written as:

$$\frac{1}{\mu} = \frac{1}{\mu\_{\text{max}}} + \frac{K}{\mu\_{\text{max}}} \frac{s\_0 - s}{s} \tag{19}$$

For a completely mixed system 1/µ= θ and 1/µm= θm. Therefore

$$
\Theta = \frac{1}{\mu\_{\text{max}}} + \frac{\text{K}}{\mu\_{\text{max}}} \frac{\text{s}\_{\text{0}} - \text{s}}{\text{s}} \tag{20}
$$

here

400 Biogas

This system consists of 4 components which are hydrolysis reactor, liquid-solid separator, storage tank and methanogenesis reactor. The dimensions of those four components are

**separator** 

Influent for the hydrolysis reactor is banana stem waste slurry. The type of reactor is anaerobic batch reactor. Reactor volume is 20 litre. Initial concentration of the biomass in teh reactor is 5000 mg/l which is mixed culture from acclimatized banana plantation soil. Inlet and outlet from this reactor is drawn manually (Scharer et al. 1981; Gavala et al. 2003).

The second component is solid-liquid separator. The tank volume is 10 litre. The function of this tank is to separate the solid and liquid from CRR effluent. The effluent from CRR will be sediment at the bottom of conical shaped separator tank. Sludge from separator will be recycled back to CRR and the liquid from separator will be transferred to storage tank

BPR with 10 litre volume function as biogas production reactor. This reactor is anaerobic fixed bed reactor and contained plastic media for biomass support. The initial biomass concentration in the reactor is 5000 mg/l which is acclimatized mixed culture from banana

COD concentration was spectrophotometrically analyzed using a HACH spectrophotometer and methods as in Spectrophotometric Instrument Manual. Gas

2 2 - 2

**Storage tank BPR** 

Lower-6

**CRR Solid-liquid** 

Volume (l) 20 10 10 10 Diameter (in) 12 9 10 Upper-10

Length (in) 15 16 10 35

Relieve 0.2 0.2 - 0.2

**2.3 Two-stages biogas production system description** 

listed in Table 1. Detailed of each component are as follows:

Table 1. Component dimension in two-stages biogas production

The storage tank with 10 litre function as storage for BPR influent.

**2.3.1 Bioreactor description** 

Maximum pressure(bar)

**2.3.2 Hydrolysis reactor** 

**2.3.3 Solid-liquid separator** 

**2.3.5 Biogas Production Reactor (BPR)** 

(Batstone et al. 2002).

**2.3.4 Storage tank** 

plantation soil.

**2.4 Analytical methods** 

S is substrate total effluent

The kinetic parameters, μmax and K, were calculated with the aim of studying possible inhibition phenomena. Using the least squares method, the values for the kinetic parameters μmax and K can be obtained from the intercept and the slope of the adjusted lines. Thus, according to Eq. 20, μmax = 1/intercept, and K = slope/intercept. Through linear regression T vs S value of μmax and K could be determined (Fig. 2). Here

T=θ

and

$$\mathbf{S} = \mathbf{S}\_o \mathbf{\cdot} \mathbf{S} / \mathbf{S} \mathbf{\cdot}$$

In this study the value of μmax and K calculated were 0.111 d-1 and 0.330 g/g respectively.

The kinetics of methane fermentation as proposed by Chen and Hashimoto (1978) is described by

$$\mathcal{S} = \frac{\mathcal{S}\_0 \mathcal{S}\_0}{\theta} \left| 1 - \frac{\mathcal{K}}{\mu\_{\text{max}} \theta - 1 + \mathcal{K}} \right| \tag{21}$$

Kinetics of Biogas Production from Banana Stem Waste 403

St Y 0.033 0.085 0.028 0.081 0.047 0.088 0.090 0.101 0.068 0.090 0.081 0.100 0.068 0.100 0.068 0.101 0.071 0.098 0.046 0.090 0.051 0.092 0.105 0.107 0.105 0.109

Table 2. Values of Y and St for linear regression to determine B0

Fig. 3. Determination of B0

Fig. 2. Determination of μmax and K

Equation (21) states that for a given substrate loading rate (S0/θ), the daily volumetric methane production depends on the biodegradability of the wastewater (B0) and the kinetic parameters θ and K (Yeoh, 1997). These values are calculated from the values of methane volume (Table 2), taking into account that the influents used have a COD of 2000 mg/l. Least squares method were used to determine the intercept B0. Through linear regression Y vs St the value of B0 could be determined. Here Y=δ and

$$\mathbf{St} = \frac{\mathbf{s}\_o}{\theta} \left| 1 - \frac{\mathbf{K}}{\mu\_{\text{max}} \theta - 1 + \mathbf{K}} \right|.$$

Fig. 3 shows the regression to determine B0. The value of B0 from this study is 0.326 l methane/g COD.

Equation (21) states that for a given substrate loading rate (S0/θ), the daily volumetric methane production depends on the biodegradability of the wastewater (B0) and the kinetic parameters θ and K (Yeoh, 1997). These values are calculated from the values of methane volume (Table 2), taking into account that the influents used have a COD of 2000 mg/l. Least squares method were used to determine the intercept B0. Through linear regression Y

� �� � �

Fig. 3 shows the regression to determine B0. The value of B0 from this study is 0.326 l

����������.

Fig. 2. Determination of μmax and K

methane/g COD.

vs St the value of B0 could be determined. Here Y=δ and

St= ��


Table 2. Values of Y and St for linear regression to determine B0

Fig. 3. Determination of B0

Kinetics of Biogas Production from Banana Stem Waste 405

COD in X Xe δ 0.835 0.230 0.200 0.085 0.780 0.410 0.180 0.081 1.058 0.390 0.250 0.088 2.604 0.480 0.250 0.101 2.262 0.320 0.250 0.090 1.851 0.320 0.250 0.100 1.515 0.110 0.080 0.100 1.515 0.110 0.080 0.101 1.515 0.110 0.080 0.098 1.032 0.800 0.550 0.090 2.094 0.450 0.450 0.092 2.463 0.390 0.520 0.107 2.463 0.930 0.540 0.109

Table 3. Values of influent substrate concentration (COD in), biomass in bioreactor concentration (X), biomass in effluent concentration (Xe) and daily volumetric methane

This study Banana stem waste 0.326 0.330 0.111 Gunaseelan (2004) Banana peel 0.277\* - 0.089 Faisal & Unno (2001) Palm oil mill wastewater 0.381 - 0.304 Gunaseelan(2007) Banana peel 0.322\* - -

The values of B0, μmax and K is fixed for different type of wastewater. So the Eq. (21) and values of B0, μmax and K could be used in scaling up biogas production process. The values of B0, μmax and K was compared to other research in Table 4. B0 is a good parameter to determine biodegradability of any particular waste (Torres-Castillo et al. 1995). From Table 1 it was shown that banana stem waste from this research have a good methane production potential and comparable with other agricultural waste such as POME and sugar cane waste. μmax is microorganism maximum growth rate and the value of μmax in this study is

Treatment plant wastewater

& fried onion

Table 4. Comparison of kinetics parameter to other research

Vegetable product-peas Vegetable product-leek

**B0** 

0.36 0.36

**(l methane/ g COD)** 

**K (g/g) μmax**

0.366 0.079 1.16



production(δ)

**Study Substrate** 

Zhao & Viraraghavan

Maya-Altamira et. al

\*l methane/g VS added

(2004)

(2008)

The kinetic values corresponding to the substrate used (banana stem waste) is of the same order of magnitude with those obtained in the mesophilic anaerobic digestion process of some solid wastes such as vegetable waste, banana peel and palm oil (Table 4). From the kinetic constants (K, µmax & B0), the theoretical daily volumetric methane production values (δ) were calculated by using Eq. (21). It can be seen that the experimental results were reproduced with errors equal to or less than 5% in all cases. As can be seen in Table 3 and Fig. 4, the values of δ increased when influent substrate concentration (COD in) increased. Therefore, the kinetic parameters were found to be influenced by the influent substrate concentration. When the influent substrate concentration increased from 0.835 to 2.463 g COD/l, the δ values also increased from 0.085 to 0.109 l methane/g COD. The mixed culture in biomass also gives effect to methane production. As can be seen in Table 3, when the biomass in the bioreactor concentration (X) increased from 0.23 g/l to 0.93 the δ increased from 0.085 to 0.109 was, therefore, multiplied by a factor of 1.3. A similar behaviour was observed in the anaerobic digestion process of traditional olive mill wastewaters (Borja et al., 1995). In this work, the minimum hydraulic retention time, θmin (days), at which the washout of the micro-organisms occurs was: 9.04 days. These values were calculated by using Eq. (13) and taking into account that this retention time is numerically equal to the reciprocal of the maximum micro-organisms growth rate (μmax).

Fig. 4. Graph for influent substrate concentration (COD in), biomass in bioreactor concentration (X) and biomass in effluent concentration (Xe)

The kinetic values corresponding to the substrate used (banana stem waste) is of the same order of magnitude with those obtained in the mesophilic anaerobic digestion process of some solid wastes such as vegetable waste, banana peel and palm oil (Table 4). From the kinetic constants (K, µmax & B0), the theoretical daily volumetric methane production values (δ) were calculated by using Eq. (21). It can be seen that the experimental results were reproduced with errors equal to or less than 5% in all cases. As can be seen in Table 3 and Fig. 4, the values of δ increased when influent substrate concentration (COD in) increased. Therefore, the kinetic parameters were found to be influenced by the influent substrate concentration. When the influent substrate concentration increased from 0.835 to 2.463 g COD/l, the δ values also increased from 0.085 to 0.109 l methane/g COD. The mixed culture in biomass also gives effect to methane production. As can be seen in Table 3, when the biomass in the bioreactor concentration (X) increased from 0.23 g/l to 0.93 the δ increased from 0.085 to 0.109 was, therefore, multiplied by a factor of 1.3. A similar behaviour was observed in the anaerobic digestion process of traditional olive mill wastewaters (Borja et al., 1995). In this work, the minimum hydraulic retention time, θmin (days), at which the washout of the micro-organisms occurs was: 9.04 days. These values were calculated by using Eq. (13) and taking into account that this retention time is numerically equal to the

reciprocal of the maximum micro-organisms growth rate (μmax).

Fig. 4. Graph for influent substrate concentration (COD in), biomass in bioreactor

concentration (X) and biomass in effluent concentration (Xe)


Table 3. Values of influent substrate concentration (COD in), biomass in bioreactor concentration (X), biomass in effluent concentration (Xe) and daily volumetric methane production(δ)


\*l methane/g VS added

Table 4. Comparison of kinetics parameter to other research

The values of B0, μmax and K is fixed for different type of wastewater. So the Eq. (21) and values of B0, μmax and K could be used in scaling up biogas production process. The values of B0, μmax and K was compared to other research in Table 4. B0 is a good parameter to determine biodegradability of any particular waste (Torres-Castillo et al. 1995). From Table 1 it was shown that banana stem waste from this research have a good methane production potential and comparable with other agricultural waste such as POME and sugar cane waste. μmax is microorganism maximum growth rate and the value of μmax in this study is

Kinetics of Biogas Production from Banana Stem Waste 407

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considered low compared to other research. This is because the value of kinetic parameter μmax and K is dependent on each other. High value of K could lower down the value of μmax and vice versa. However the value of μmax reported in this research is still within acceptable range.
